Fix mailbox Exception type.

Making mailbox raised exception more precise.
May be the 1st of an improved mailbox logging patches.

Change-Id: Id8baf1121aaf351b029945972d5e87dbfa6a0bd5
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/6617
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Haim Ateya <hateya@redhat.com>